Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or another lean burn engine offer the benefit of increased fuel economy, however, control of NOx emissions in such systems is complicated due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant, such as urea, into the exhaust gas mixture entering the catalyst, are know to achieve high NOx conversion efficiency. For future, more stringent emission regulations, most diesel vehicles also require a particulate filter to trap and burn particulate matter (PM).
A typical lean exhaust gas aftertreatment system is described in WO 99/39809 and includes an oxidation catalyst for oxidizing NO, CO and HC in the engine exhaust gas mixture followed by a particulate filter downstream. The particulate filter stores particulate matter, such as soot, and requires periodic regeneration at high temperatures. The particulate filter has an SCR catalyst downstream of it.
The inventors herein have recognized several disadvantages with the prior art approach. Namely, because the SCR catalyst is located farthest downstream of the engine in this configuration, there is a significant delay in achieving light-off temperatures thus negatively affecting the system's NOx conversion efficiency. Further, since the particulate filter has to be regenerated at high temperatures, separate cooling means have to be employed to cool down the hot exhaust gas exiting the particulate filter in order to prevent irreversible thermal damage to the SCR catalyst. Still another disadvantage of the prior art system is that not all ammonia may be utilized in the SCR catalyst, and may therefore be emitted into the atmosphere.
In accordance with the present invention, a system that achieves effective control of NOx and particulate matter emissions in a lean burn engine, such as a diesel engine, while overcoming the disadvantages of the prior art is presented. The system includes an oxidation catalyst, a selective catalytic reduction (SCR) catalyst coupled downstream of said oxidation catalyst; and a particulate filter coupled downstream of said SCR catalyst. This placement of the emission system components provides improved NOx conversion efficiency, reduced tailpipe emissions of ammonia, and minimizes fuel economy penalty for the particulate filter regeneration.
In one embodiment of the present invention, the particulate filter is a catalyzed particulate filter containing a washcoat of a precious metal such as platinum.
In another embodiment of the present invention, an ammonia clean-up catalyst is coupled between the SCR catalyst and the particulate filter to selectively convert any ammonia slipped from the SCR catalyst into nitrogen.
In yet another embodiment of the present invention, a second oxidation catalyst is coupled between the SCR catalyst and the particulate filter to generate additional heat for PM combustion in the particulate filter.
An advantage of the present invention is that improved NOx conversion efficiency is achieved by placing the SCR catalyst higher upstream than the prior art systems, due to faster SCR catalyst warm-up and its higher operating temperatures.
Another advantage of the present invention is that placing the particulate filter farthest downstream eliminates the risk of the thermal damage to other emission system components during filter regeneration.
Yet another advantage of the present invention is that ammonia that slipped from the SCR catalyst is oxidized in the particulate filter, thus resulting in reduced tailpipe ammonia emissions.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.